This invention relates to a method and apparatus for suppressing azimuthal modes in a laser cavity. More specifically, an optical system is employed which rotates a portion of the output laser beam and feeds this beam portion back into the laser cavity. A diffractive coupling occurs between the feedback beam and the output beam which reduces unwanted modes, particularly the azimuthal modes. The resultant far-field distribution is considerably improved.
Mode control problems existing in lasers are of the temporal type and involve phase coherence. However, large size lasers are now being built in which a different type of problem has emerged. This is caused by imperfect diffractive coupling of the beam through the output optics of the system and results in the formation of azimuthal modes.
Analogous to the case of imperfect temporal modes in a solid state laser, the formation of azimuthal modes causes a deterioration in the far-field pattern of the beam.
Azimuthal mode control becomes pronounced for laser parameters A.sup.2 /.lambda. L .gtoreq. about 50-100, where A represents the limiting radius of the output aperture, .lambda. is the wave length of the laser beam and L is the path length of the particular mode. This applies whether the laser medium is a solid, liquid, gas or plasma.